Evolution of the Josephine Peridotite Shear Zones: 1. Compositional Variation and Shear Initiation

Abstract

Shear localization in the upper mantle, a necessity for plate tectonics, can have a number of causes, including shear heating, the presence of melt, the development of a strong crystal preferred orientation, and the presence of water. The Josephine Peridotite of southwestern Oregon contains shear zones that provide an excellent opportunity to examine the initiation of shear localization. These shear zones are relatively small scale and low strain compared to many shear zones in peridotite massifs, which typically have extreme grain size reduction indicating extensive deformation. We use major, trace, and volatile element analyses of a large suite of harzburgites from the Fresno Bench shear zones to evaluate the mechanisms leading to shear localization. Lithological evidence and geochemical transects across three shear zones show a complex history of melting, melt addition, and melt‐rock interaction. The distribution of aluminum and heavy rare earth elements across the shear zones suggest that melt flow was focused in the centers of the studied shear zones. Water concentrations in orthopyroxene grains of 180–334 ppm H2O indicate a comparatively high degree of hydration for nominally anhydrous minerals. The correlation of water with aluminum and ytterbium in orthopyroxene is consistent with a melt source for this hydration, suggesting that water equilibrated between the melt and peridotite. The presence of melt and hydration of the host rock provide mechanisms for initial weakening that lead to localized deformation.

The potential for aqueous fluid-rock and silicate melt-rock interactions to re-equilibrate hydrogen in peridotite nominally anhydrous minerals

“…For example, Warren et al (2009) estimated that peridotites from the ultraslow spreading Southwest Indian Ridge can take up to ~13 Myr to cool from 1200°C to 0°C. During this long uplift history, NAMs are susceptible to melt-rock or aqueous fluid-rock interactions that can diffusively modify their water contents (e.g., Kumamoto et al 2019). The fast reequilibration times in our models compared to these long cooling histories suggest that peridotite…”

“…The water contents of NAMs in peridotites exposed on the ocean floor and as massifs on land are often interpreted to record mantle water left behind after partial melting (e.g., Warren and Hauri 2014;Kumamoto et al 2019).…”

“…Measurements using FTIR offer the benefit of site-specific distributions of H + storage and diffusion pathways, but analytical resolution depends on the choice of polarized vs. unpolarized measurements, oriented vs. unoriented grains, and sample thickness (e.g., Bell et al 2003;Aubaud et al 2009;Stalder et al 2012;Rossman 2013a, 2013b;Withers et al 2012). For abyssal, opholitic, and orogenic peridotites, NAM water contents are more commonly measured by SIMS (e.g., Warren and Hauri 2014;Kumamoto et al 2019). Thus, we utilize the analytical uncertainty from Warren and Hauri (2014) for SIMS measurements of olivine, opx, and cpx to adjust our modeled 100% re-equilibration timescales.…”

“…DOI: https://doi.org/10.2138/am-2021-7435. http://www.minsocam.org/Kumamoto et al (2019) across shear zones in the Josephine Peridotite identified variations in LREE contents over spatial scales of several meters, inferred to be a result of melt-rock interaction. The water content in orthopyroxene is also variable across the transects, but decoupled from other trace element variations.…”

Evolution of the Josephine Peridotite Shear Zones: 2. Influences on Olivine CPO Evolution

“…Schematic orientations of the expected low-angle misorientation axes are plotted in the misorientation-axis inverse pole figures (MIPFs). After Skemer et al (2012). have formed in the shallow mantle at <30-km depth and temperatures of 900-1200°C, based on major element geothermometry of the peridotite and syndeformational gabbroic dikes (Harding, 1988;Kelemen & Dick, 1995;Kumamoto et al, 2019;Loney & Himmelberg, 1976;Skemer et al, 2010).…”